1. Field of the Invention
The invention relates to electronic autocorrelating signal systems employing maximal length PRC coding, and more particularly, to PRC radar systems.
2. Description of the Prior Art
In the prior art the concept of autocorrelation of a given pseudorandom coded (PRC) waveform against the same waveform provided through another route, as for example, as a radar echo, has been widely and advantageously used, not only in the radar arts, but in certain other specialized areas.
For a background in connection with the general technique of pseudo-noise or pseudo-random coded system technology, the reader is referred to the technical literature. In particular, the text "Radar Handbook", by Merrill I. Skolnik (McGraw-Hill, 1970) describes the state of this art in respect to radar systems relatively currently, and provides extensive bibliographic references. See Chapter 20, Section 20.5 of that text, in particular. In general, the pseudo-random coded waveform is generated by digital techniques, and is imposed as a modulation on a radio-frequency carrier which may be transmitted toward distant points or targets. Received echoes as a result of those transmissions bear the same, usually bi-phase modulation, corresponding to a pseudo-random sequence of "ones" and "zeroes". The autocorrelation process is then applied, and such systems are known to provide certain distinct advantages, among these being implementation of pulse compression, and the provision of good signal-to-noise ratios in an arrangement which is relatively immune to certain types of interference.
Additional references for an understanding of the basic techniques involved as background of the present invention may be found in two other texts, namely; "Radar Design Principles" by F. Nathanson, (Chapter 12, in particular) and "Modern Radar", by R. S. Berkowitz, (John Wiley, publisher), (Chapter 4, in particular).
The use of the pseudo-random coded technique for a non-radar application is described in U.S. patent application, Ser. No. 458,808, filed Apr. 8, 1974, entitled: "Large Area Motion Sensor Using Pseudo-Noise Technique" and now abandoned. That application is assigned to the assignee of the present application.
PSEUDO-RANDOM CODED SYSTEMS MAY BE OF THE PULSE TYPE, OR MORE COMMONLY, ARE OF THE CW type. The term maximal length sequence applies to CW systems in which one transmitted pseudo-random coded word is transmitted repetitively without hiatus between the end of one word and the beginning of the next. It is this type of system to which the present invention is particularly applicable, and the description hereinafter presumes that type of system.
A typical maximal length sequence generator produces a code of length L binary bits. Quite commonly, a shift-register/feedback arrangement is employed, and if n is the number of stages of the shift register, the word-length may be described as L= 2.sup.n - 1. Accordingly, a three-stage sequence generator produces a code of L= 2.sup.3 - 1= 7 bits, and a five-stage sequence generator produces a code of L= 2.sup. 5 - 1= 31 bits. These sequences are repetitive and the individual bits are either "ones" or "zeroes". In a five-stage generator, the number of ones is given by 2.sup.n /2 = 32/2 = 16, and the number of zeroes is given by (2.sup.n /2 )- 1= 16- 1= 15, thus the number of logical "zeroes" is always one less than the number of logical "ones".
The average value of the waveform of such a sequence is therefore 1/L, i.e., 1/7 for the three stage coder example and 1/31 for the 31 bit codeword generated by the five-stage sequence generator. The DC term of power spectrum may be thought of as (1/L).sup.2, or 1/961 for the five-stage coder.
Typical examples of prior art three and five-stage maximal length sequence generators are shown in the drawings and will be appropriately described hereinafter in connection with the description of the preferred embodiments. Although those prior art configurations do not include the present invention, a clear understanding of them is necessary in order to appreciate the significance of the present invention.
In accordance with the foregoing, it wil be realized that prior art pseudo-random sequence coders inherently generate a code which, although optimum in certain applications, does, nevertheless, correlate out with a residue on either side of the autocorrelation peak which is other than zero. In the typical ranging situation using a pseudo-random coded technique, the received code "slips by" the transmitted code as a result of relative sending and receiving station range changes,the maximum correlator output occurring when code words are perfectly matched. Thus, when the code is multiplied by itself and integrated (the autocorrelation process), the autocorrelated value reaches a peak when the codes are aligned exactly, but for each integer bit of slippage between the two multiplied codes, the value is -1/L that of the peak value.
For certain precision range measuring applications, the fact that there is a residue on either side of the autocorrelation peak tends to be disadvantageous since the autocorrelation peak is thereby less sharply defined than would be the case if there were no residue.
The manner in which the present invention deals with the said autocorrelation residue problem will be understood as this description proceeds.